This invention relates to AC electric motors and generators of the synchronous type, specifically to the incorporation of the excitation system internally in the machine itself, eliminating the need for brushes and a separate exciter.
A synchronous type of AC machine, whether it be a motor or a generator, has fixed magnetic poles on its rotor. These poles are typically excited by providing a DC current, called the field current or the exciting current, through the field windings which are located on the rotor. There are also synchronous machines constructed with permanent magnets on the rotor. These machines do not need field current and the field flux has a fixed magnitude which is not easily adjustable. This invention does not include such machines, but is applicable only to machines which are excited by a DC field current.
The arrangement for providing the DC exciting current is called the excitation system. Typically the source of DC is another machine called the exciter, which is coupled to the synchronous machine and rotates with it. Earlier exciters were DC generators. Such machine plus exciter systems needed several brushes because the DC exciter also had to have brushes on its commutator. Later, totally brushless excitation systems became popular, which used an AC generator as the exciter. In such exciters the windings where AC is generated are located on the rotor. The generated AC is converted to DC by a diode rectifier circuit mounted on the rotor and fed directly to the field of the synchronous machine, thereby totally eliminating the need for any brushes or sliding contacts. The adjustment of the field current is made by adjustment of the field current of the exciter machine, whether it is an AC exciter or a DC exciter. Since the field of the exciter machine is on its stationary side in both types of exciters, the field current adjustment can be done from the stationary frame in both cases.